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8.1.1 Active Characters

To fully understand where proper quotation is important, you first need to know what the special characters are in Autoconf: ‘#’ introduces a comment inside which no macro expansion is performed, ‘,’ separates arguments, ‘[’ and ‘]’ are the quotes themselves1, ‘(’ and ‘)’ (which M4 tries to match by pairs), and finally ‘$’ inside a macro definition.

In order to understand the delicate case of macro calls, we first have to present some obvious failures. Below they are “obvious-ified”, but when you find them in real life, they are usually in disguise.

Comments, introduced by a hash and running up to the newline, are opaque tokens to the top level: active characters are turned off, and there is no macro expansion:

     # define([def], ine)
     ⇒# define([def], ine)

Each time there can be a macro expansion, there is a quotation expansion, i.e., one level of quotes is stripped:

     int tab[10];
     ⇒int tab10;
     [int tab[10];]
     ⇒int tab[10];

Without this in mind, the reader might try hopelessly to use her macro array:

     define([array], [int tab[10];])
     ⇒int tab10;

How can you correctly output the intended results2?


[1] By itself, M4 uses ‘`’ and ‘'’; it is the M4sugar layer that sets up the preferred quotes of ‘[’ and ‘]’.

[2] Using defn.